if you have vsync enabled, doublebuffering stalls when you do the swap, taking away cpu and gpu cycles to wait till you can swap the image to screen.
you don't have this with trible buffering (well, you still have, but only for a minimal time).
so you're not limited to 60fps or similar like before when using vsync. the screen will still display only 60fps. but that way, you can get 100% cpu power back again. wich can be important.